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Raccoons, Parasites Have Bioterrorism Potential

The ubiquitous raccoon can carry Baylisascaris procyonis, a parasite that can cause life-threatening infections. (U. S. Fish and Wildlife Service/photo by John and Karen Hollingsworth.)

To solidify defenses against potential agents of bioterrorism, it makes sense to consider exotic organisms that tend to be ignored, even though some of them remain surprisingly accessible and have been tested for such purposes—albeit by amateurs. Some deliberate incidents involving parasitic roundworms three decades ago are a reminder that bioterrorism risks sometimes come in strange packages.

The introduction of Salmonella typhimurium into foods being served in restaurants in and around Dalles, Ore., in September 1984 by the Bhagwan Shree Rajneesh cult is often called the first modern act of bioterrorism in North America. It was not.

Fourteen years earlier, on 1 February 1970, four university students in Montreal, Quebec, Canada, were infected—deliberately—with a massive dose of roundworm eggs from the parasite Ascaris suum. The students became acutely ill with pulmonary infiltrates, asthma, and eosinophilia. After being treated with prednisone and piperazine, all four recovered. A parasitology graduate student was later identified as the attacker. This unusual assault with a common pig parasite, although reported two years later in the New England Journal of Medicine (286:965-970, 1972), has largely faded from view. "It happened so many years ago that I forgot about it," recently noted one of the original investigators, Lewis Perelmutter, formerly of Queen Elizabeth Hospital, McGill University, Montreal.

Yet the incident is worth reexamining, particularly for the ease of the attack and the unusual choice of pathogen. Two more recent reports about a similar parasitic roundworm, Baylisascaris procyonis, reinforce the notion that we remain especially vulnerable to such obscure, but readily accessible pathogens. This latter roundworm is responsible for 12 scattered cases of infection in the United States since 1981, including two deaths, according to a report in Morbidity and Mortality Weekly Report early this year (50:1153-1155, 2002).

Although the case numbers are few, Frank Sorvillo of the University of California Los Angeles School of Public Health and coworkers state in Emerging Infectious Diseases (8:355-359, 2002) that B. procyonis is an underrecognized, emerging helminthic zoonotic that is a potential bioterrorism agent. Of particular note, human infections can lead to devastating neurologic sequelae or death.

B. procyonis is a nematode that parasitizes the intestinal tracts of North American raccoons, whose droppings release huge quantities of infectious B. procyonis eggs. Raccoons are unusual in that they tend to defecate in specific sites, including fallen trees, rocks, and stumps in the wild, and on woodpiles, decks, walls, steps, and flat rooftops, in suburban settings. Such sites provide likely points of human contact with B. procyonis.

The worm eggs are extremely hardy and, provided with moisture, will remain infectious for years. They are resistant to most disinfectants, including sodium hypochlorite and quaternary ammonium solutions. Raccoons shed as many as 250,000 eggs per gram of feces, and as few as 5,000 eggs may serve as an infective dose. In effect, raccoon droppings may be as dangerous as the often rabies-infected animals themselves.

The full spectrum of B. procyonis disease is unknown. Eggs ingested by humans and other intermediate hosts escape from the intestine, and the larvae migrate to other tissues, including the brain and eyes. Host inflammatory responses likely contribute to the subsequent neurologic and ocular damage. There is no effective therapy. Various antihelminthic drugs, including ivermectin, and antiinflammatory agents have been used to treat the infection, but with little obvious success. There is also no vaccine, no standardized diagnostic reagents, and no active federal research on this emerging pathogen, with which few practicing physicians are familiar.

Acquired by accident or intent, B. procyonis causes life-threatening, long-lasting infections potentially leading to permanent damage or death. As a potential tool of bioterrorism, B. procyonis has a number of features that make it more dangerous than it first looks. It is a largely unknown pathogen, yet is readily available in rural and suburban areas of the United States. It is easy to isolate and maintain, and its low-dose infectivity and overall hardiness give it potential for being slipped into postproduction foods and postfiltration water systems. Given their history and biology, this and similar parasites need to be put back on the bioterrorism countermeasures radar screen.

Edward McSweegan
Edward McSweegan is a microbiologist in the Washington, D.C., area.